Interpretive Summary: Aflatoxins are natural toxins produced by the two common molds, Aspergillus flavus and A. parasiticus. When these fungi invade crops and produce aflatoxins, they render the crops unsalable. The carcinogenic property of aflatoxins has stimulated many scientists to study in detail how these toxins are produced. ARS efforts are focused on the elucidation of the mechanism of aflatoxin production. In order to better understand how aflatoxin is produced ARS scientists cloned a gene (estA) for an enzyme (esterase) and an additional copy of this gene. The results revealed that only one copy of the estA gene is expressed. The other copy is not expressed, due possibly to its chromosomal location. The understanding of the estA gene function and the non-functionality of the second copy may provide insight into evolution biology and how to interrupt the process of aflatoxin production in fungi that make aflatoxin and that infest U.S. crops, thus reducing aflatoxin contamination in food and feed.

Technical Abstract:
Within the aflatoxin pathway gene cluster characterized earlier, we have identified a estA1 encoding an esterase from wild type strain of Aspergillus parasiticus SRRC 143. The 1,500 bp genomic DNA and 945 bp cDNA sequences were determined. Outside of the aflatoxin pathway gene cluster, an additional copy of the estA gene (named estA2) was cloned from the same A. parasiticus strain SRRC 143. There are only 8 substitutions within the 314 amino acid residues of estA2 gene product compared with that of estA1 and no apparent defect was identified in the estA2. The estA1 gene is a homolog of the stcI gene identified in A. nidulans involved in the biosynthesis of sterigmatocystin. RT-PCR experiments demonstrated that only the estA1 gene, which is located within the aflatoxin pathway gene cluster, is expressed; no expression of the estA2 gene was detected under both aflatoxin conducive and non-conducive conditions. Possible reasons for the preferential expression of the estA1 over the estA2 gene have been discussed.